Preterm birth (defined as delivery before 37 weeks of gestation) is the most important cause of neonatal morbidity and mortality in the developed world and its incidence continues to rise. Current treatments have limited efficacy because the fundamental mechanisms leading to labor-whether term or preterm-are not known. Since successful pregnancy depends on multi-layered tolerance between the mother and the fetus, pregnancy complications may involve a breakdown in such mechanisms. We propose to examine the novel hypothesis that infection or inflammation during pregnancy results in the activation of maternal T cells specific for fetal or placental antigens, and of fetal T cells speciic for maternal antigens, ultimately resulting in rejection and preterm delivery. We will use mouse models and patient samples to test this hypothesis. In our mouse model, we have previously shown that fetal intervention (a model of sterile inflammation) activates maternal T cells that are specific for fetal antigens and that this activation can cause selective demise of allogeneic fetuses. This finding is consistent with other reports of T cell activation and fetal loss during Listeria infection in pregnant mice. We will now delineate the cellular mechanisms of these findings using a novel Nur77-Foxp3 double reporter mouse that tracks T cell receptor engagement in both effector and regulatory T cells. In our patient samples, we have developed robust assays to enumerate allospecific T cells to examine their possible contribution to clinical preterm labor (PTL). We made the surprising discovery that T cell proliferation in maternal blood is blunted during pregnancy but that fetal (cord blood) T cells are activated in patients with PTL secondary to infection. We propose to examine whether maternal T cells in the maternal tissue layer in contact with the placenta (the decidua) become activated during PTL, and the mechanisms leading to early activation of fetal T cells, in patients with PTL secondary to infection or fetal intervention. The latter is commonly performed at our institution to treat fetuss with congenital anomalies but results in a significantly increased rate of PTL. In Aim 1, we will analyze effector and regulatory T cell activation in mice in the context of two pregnancy complications: fetal intervention and intrauterine infection. In Aim 2, we will examine maternal immune responses against fetal and placental antigens in patients with PTL using a novel functional assay to track allospecific T cells and explore changes in tolerance mechanisms that may be perturbed during inflammation. In Aim 3, we will examine fetal T cell responses in patients with PTL to understand the mechanisms of accelerated T cell maturation in this population. Our short-term goal is to understand the possible contribution of maternal and fetal T cells to the pathogenesis of PTL. Our long-term goals are to understand mechanisms leading to term and preterm labor, to gain insights into fetal and neonatal T cell development and its consequences for neonatal health, and, ultimately, to develop targeted therapies to impede T cell activation in patients at risk for PTL.